Surface light emitting semiconductor laser element

ABSTRACT

A surface light emitting semiconductor laser element, comprises a substrate, a lower reflector including a semiconductor multi-layer disposed on the substrate, an active layer disposed on the lower reflector, an upper reflector including a semiconductor multi-layer disposed on the active layer, a compound semiconductor layer having a first opening for exposing the upper reflector and extending over the upper reflector, and a metal film having a second opening for exposing the upper reflector disposed inside of the first opening and extending over the compound semiconductor layer, wherein the metal film and the compound semiconductor layer constitute a complex refractive index distribution structure where a complex refractive index is changed from the center of the second opening towards the outside. A method of emitting laser light in a single-peak transverse mode is also provided.

The subject matter of application Ser. No. 10/847,904 is incorporatedherein by reference. The present application is a divisional of U.S.application Ser. No. 10/847,904, filed May 18, 2004, which claimspriority to Japanese Patent Application No. JP2003-140181, filed May 19,2003. The present application claims priority to these previously filedapplications.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a surface light emitting semiconductorlaser element and a method of manufacture thereof. More particularly,the present invention relates to a surface light emitting semiconductorlaser element and a method of emitting laser light in a single-peaktransverse mode.

2. Description of the Related Art

A surface light emitting semiconductor laser element emits laser lightin a direction orthogonal to a surface of a substrate, and is aremarkable light source for application in various fields.

The surface light emitting semiconductor laser element has asemiconductor substrate, a pair of upper and lower reflectors, i.e.,Diffractive Bragg Reflectors (DBRs) comprising compound semiconductorshaving different refractive indices on the substrate, and an activelayer that constitutes a light emitting area between the pair ofreflectors.

Typically, the surface light emitting semiconductor laser element has apost-type mesa structure were the upper DBR has a current confinementarea. For example, Japanese Unexamined Patent Application PublicationNo. 2001-210908 discloses a surface light emitting semiconductor laserelement comprising a circular post-type mesa structure having a mesadiameter of about 30 μm obtained by dry etching the upper DBR, and acurrent confinement area within the circular post-type mesa structureformed by selectively oxidizing an AlAs layer to efficiently injectcurrent into the active layer.

Referring to the above-mentioned Japanese patent application publicationand FIG. 12, a conventional surface light emitting semiconductor laserelement comprising a post-type mesa structure will be described. FIG. 12is a sectional view showing the structure of the conventional surfacelight emitting semiconductor laser element disclosed in theabove-mentioned patent application publication.

As shown in FIG. 12, a surface light emitting semiconductor laserelement 80 has a laminated structure sequentially comprising an n-typeGaAs substrate 82, a lower diffractive bragg reflector (hereinafter“lower DBR”) 84 comprising an n-type semiconductor multi-layer, a lowerclad layer 86 comprising non-doped AlGaAs, a light emitting layer(active layer) 88, an upper clad layer 90 comprising non-doped AlGaAs,an upper diffractive bragg reflector (hereinafter “upper DBR”) 92comprising non-doped AlGaAs, and a p-type GaAs cap layer 94.

The lower DBR 84 has a semiconductor multi-layer structure including30.5 pairs of n-type Al_(0.2)Ga_(0.8)As layers and n-typeAl_(0.9)Ga_(0.1)As layers having composition gradient layers on thehetero interfaces. The upper DBR 92 has a semiconductor multi-layerstructure including 25 pairs of p-type Al_(0.2)Ga_(0.8)As layers andp-type Al_(0.9)Ga_(0.1)As layers having composition gradient layers onthe hetero interfaces.

A cylindrical mesa post 96 is formed by etching the cap layer 94, theupper DBR 92, the upper clad layer 90, the active layer 88, the lowerclad layer 86, and the lower DBR 84.

A p-type AlAs layer is formed instead of the p-type Al_(0.9)Ga_(0.1)Aslayer on the compound semiconductor layer of the upper DBR 92 at thenearest side of the active layer 88. Al contained in the p-type AlAslayer is selectively oxidized excluding a center circular area toprovide an oxidized-Al current confinement layer 98.

The p-type AlAs layer remaining on the center circular area functions asa current injection area 98A, and the oxidized-Al current confinementlayer functions as an insulation area 98B having high electricalresistance.

A SiN_(x) film 100 is formed over the mesa post 96 and the lower DBR 84.The SiN_(x) film has an opening for exposing the p-type GaAs cap layer94 provided by circularly removing the SiN_(x) film 100 on the uppersurface of the mesa post 96. A circular p-side electrode (upperelectrode) 102 is formed at the periphery of the opening. On theopposite surface of the n-type GaAs substrate 82, an n-side electrode(lower electrode) 104 is formed. The p-side electrode 102 has anextraction electrode 106.

Referring to FIGS. 13A and 13B, a method of producing the surface lightemitting semiconductor laser element 80 will now be described. FIGS. 13Aand 13B are sectional views showing steps of producing the surface lightemitting semiconductor laser element 80.

As shown in FIG. 13A, the laminated structure is formed by sequentiallylaminating the lower DBR 84, the lower clad layer 86 comprisingnon-doped AlGaAs, the active layer 88, the upper clad layer 90comprising non-doped AlGaAs, the upper DBR 92, and the p-type GaAs caplayer 94 on the n-type GaAs substrate 82.

The lower DBR 84 is produced by laminating 30.5 pairs of the n-typeAl_(0.2)Ga_(0.8)As layers and the n-type Al_(0.9)Ga_(0.1)As layershaving the composition gradient layers on the hetero interfaces. Theupper DBR 92 is produced by laminating 25 pairs of the p-typeAl_(0.2)Ga_(0.8)As layers and the p-type Al_(0.9)Ga_(0.1)As layershaving the composition gradient layers on the hetero interfaces.

Before the upper DBR 92 is formed, the p-type AlAs layer 108 is formedinstead of the p-type Al_(0.9)Ga_(0.1)As layer on the compoundsemiconductor layer of the upper DBR 92 at the nearest side of, oradjacent to the active layer 88.

As shown in FIG. 13B, the p-type GaAs cap layer 94, the upper DBR 92,the AlAs layer 108, the upper clad layer 90, the active layer 88, andthe lower clad layer 86 are partially etched using a SiN_(x) film mask110 until the upper surface of the lower DBR 84 is exposed, whereby amesa post 96 is formed.

The laminated structure having the mesa post 96 is heated at 400° C. forabout 25 minutes under steam atmosphere to selectively oxidize only thep-type AlAs layer from the side face to the center of the mesa post 96.

Thus, a current confinement layer 98 is formed. The current confinementlayer 98 has the cylindrical current confinement area 98B comprising theoxidized-Al layer, and the circular current injection area 98Acomprising the p-type AlAs layer 108 that is not oxidized and remains.The circular current injection area 98A is surrounded by the currentconfinement area 98B.

After the SiN_(x) film 100 is formed over the entire surface, theSiN_(x) film 100 on the upper surface of the mesa post 96 is circularlyremoved to expose the p-type GaAs cap layer 94 where the circular p-sideelectrode is formed. At the opposite surface of the n-type GaAssubstrate 82, the n-side electrode 104 is formed. As a result, theconventional surface light emitting semiconductor laser element 80 isprovided.

In the surface light emitting semiconductor laser element comprising thepost-type mesa structure, the current confinement layer 98 defines asection of a path for injecting a current into the active layer 88.Therefore, the current is intensively injected into the active layer 88around the current confinement area 98B, which leads to efficient laseroscillation.

Typically, the conventional surface light emitting semiconductor laserelement oscillates in a multi-mode which is a transverse mode having aplurality of peaks in the far field pattern (FFP).

When the surface light emitting semiconductor laser element islens-coupled to an optical waveguide such as an optical fiber in thecommunication field, the surface light emitting semiconductor laserelement desirably emits beams in a single-peak transverse mode, i.e., aGaussian distribution mode, in order to improve the optical connectionefficiency.

In the oxidized-type current confinement structure, the number of modesin the oscillating laser light is substantially in proportion to thesize of the current confinement layer. Therefore, when the currentinjection area in the current confinement layer is decreased, it ispossible to emit light in a single mode excited in a narrow area of theactive layer.

Accordingly, in the conventional surface light emitting semiconductorlaser element having the oxidized-type current confinement structure,when the size of the current confinement structure (current injectionarea) comprising the oxidized-Al layer is reduced, the light-emittingarea of the active layer can be decreased and light is selectivelyoscillated in the single-peak transverse mode.

In order to provide the single-peak transverse mode, the size of thecurrent confinement structure should be as small as 4 μm or less, asreported in IEEE. Photon, Tech. Lett. Vol. 9, No. 10, p. 1304, by M.Grabherr et al. However, if the size of the current confinementstructure is 4 μm or less, the following problems occur.

Firstly, the tolerance of production errors becomes limited, since thesize of the current confinement structure is extremely small. It isdifficult to produce a surface light-emitting semiconductor laserelement having a current confinement structure with a small diameterwith good controllability. Also, wafer in-plane uniformity becomes poor,resulting in significantly decreased yields.

Secondly, current flows through the decreased current injection area(AlAs layer) by one order of magnitude as compared with the typicaldevices, whereby the resistance of the element becomes high, i.e., 100Ωor more. As a result, the output as well as the current and lightemission efficiencies are lowered. In other words, since the outputdepends on the single-peak transverse mode, it is difficult to providehigh output from the surface light emitting semiconductor laser elementin the single-peak transverse mode.

Thirdly, due to the increased resistance caused by the currentconfinement, the impedance is mismatched. If the surface light emittingsemiconductor laser element is attempted to be driven at high frequency,the high frequency properties are significantly degraded. Accordingly,it is difficult to apply the surface light emitting semiconductor laserelement to light transmission driven at high frequency, as required inthe communication field.

For transverse mode control of the laser light in a surface lightemitting semiconductor laser element, Japanese Unexamined PatentApplication Publication No. 2002-359432 discloses, for example, a methodof stabilizing the transverse mode by processing a light emittingsurface. However, this publication is not directed to the stabilizationof the single transverse mode, but to the stabilization of ahigher-order transverse mode.

Japanese Unexamined Patent Application Publication No. 2001-24277discloses that a reflectance distribution is provided at a reflectingsurface opposite to a light-emitting surface to stabilize the transversemode. However, since light is injected through a substrate, it isdifficult to apply this invention to a surface light emittingsemiconductor laser element. In addition, since a proton-injection-typeis presumed, it is difficult to apply this invention to theoxidized-type current confinement structure.

Japanese Unexamined Patent Application Publication No. 9-246660discloses a method of stabilizing the transverse mode by disposing alens structure comprising a circular diffraction grating within a laser.However, the process is complicated because a compound semiconductorlayer should be re-grown. There are both technical and economicalproblems.

As described above, using the conventional technique, it is difficult toprovide a surface light emitting semiconductor laser element that emitslaser light in the single-peak transverse mode.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide asurface light emitting semiconductor laser element that emits stablelaser light in a single-peak transverse mode. It is another object ofthe present invention to provide a surface light emitting semiconductorlaser element that emits stable laser light in a high-order mode, and amethod of producing the same.

By repeating various research experiments, the present inventorsdiscovered that the oscillating transverse mode of a surface lightemitting semiconductor laser element is affected not only by the size ofthe current confinement structure, as described above, but alsosignificantly by the optical structure of an upper surface on a mesapost that acts as a light-emitting surface. In other words, thetransverse mode significantly depends on a refractive index distributionand a shape of an electrode.

Various surface light emitting semiconductor laser elements havingdifferent upper surfaces or shapes of the mesa post have been trialmanufactured to determine the relationship between the upper surfacestructure of the mesa post and the transverse mode. As a result, it wasfound that the transverse mode is greatly affected by the structure ofan electrode on a contact layer as well as the shape, the refractiveindex, and the film thickness of a semiconductor layer, i.e., a contactlayer, on the upper surface of the mesa post.

Through intensive studies, the present inventors discovered that asurface light emitting semiconductor laser element for emitting laserlight in a single-peak transverse mode can be provided by a structurecomprising a contact layer having a first opening for exposing an upperDBR and extending over the upper DBR, an electrode formed of a metalfilm having a second opening disposed inside of the first opening forexposing the upper DBR, and an insulation film between the contact layerand the electrode having a third opening disposed outside of the firstopening for exposing the contact layer, as shown in FIG. 14A. In thesurface light emitting semiconductor laser element, the impurityconcentration in a current injection area is high, and uniformity of thecurrent injection density at this area is improved.

In the structure on the upper DBR described above, a peripheral regionof the second opening in the electrode, a peripheral region of the firstopening in the contact layer, and a peripheral region of the thirdopening in the insulation film constitute a complex refractive indexdistribution structure where a complex refractive index is changedisotropically from the center of the second opening towards the outside.A single-peak transverse mode can be provided by the complex refractiveindex distribution structure.

In other words, the contact layer and the electrode constitute thecomplex refractive index distribution structure.

FIG. 14A is a schematic sectional view showing a main part of a surfacelight emitting semiconductor laser element according to one embodimentof the present invention. FIG. 14B is a schematic sectional viewillustrating a function of the main part shown in FIG. 14A.

One aspect of the present invention is to provide a surface lightemitting semiconductor laser element, comprising:

a substrate,

a lower reflector including a semiconductor multi-layer disposed on thesubstrate,

an active layer disposed on the lower reflector,

an upper reflector including a semiconductor multi-layer disposed on theactive layer,

a compound semiconductor layer having a first opening for exposing theupper reflector and extending over the upper reflector, and

a metal film having a second opening for exposing the upper reflectordisposed inside of the first opening and extended over the compoundsemiconductor layer,

wherein the metal film and the compound semiconductor layer constitute acomplex refractive index distribution structure where a complexrefractive index is changed from the center of the second openingtowards the outside.

According to the surface light emitting semiconductor laser element, inthe complex refractive index distribution structure, the complexrefractive index is changed isotropically from the center of the secondopening towards the outside. The single-peak transverse mode can be moreeasily provided.

Another aspect of the present invention is to provide a surface lightemitting semiconductor laser element, comprising:

a substrate,

a lower reflector including a semiconductor multi-layer disposed on thesubstrate,

an active layer disposed on the lower reflector,

an upper reflector including a semiconductor multi-layer disposed on theactive layer,

a compound semiconductor layer having a first opening for exposing theupper reflector extended over the upper reflector, and

a metal film including an annular film and an island-like film, theannular film having a second opening for exposing the upper reflectorbeing disposed inside of the first opening, the annular film extendingover the compound semiconductor layer, and the island-like film beingdisposed like islands on the upper reflector within the second opening,

wherein the metal film and the compound semiconductor layer constitute acomplex refractive index distribution structure where a complexrefractive index is changed from the center of the second openingtowards the outside.

In preferable embodiments of the above-mentioned aspects, the surfacelight emitting semiconductor laser element further comprises a thirdopening disposed outside of the first opening for exposing the compoundsemiconductor layer, and an insulation film interposed between thecompound semiconductor layer and the metal film, and the metal film, thecompound semiconductor layer, and the insulation film constitute acomplex refractive index distribution structure where a complexrefractive index is changed from the center of the second openingtowards the outside.

In specific embodiments of the above-mentioned aspects, the metal filmconstitutes an electrode, and the compound semiconductor layerconstitutes a contact layer in ohmic contact with the metal film. Acurrent injection area formed at a center of a current confinement layeris disposed under the first opening.

The surface light emitting semiconductor laser element according to oneaspect of the present invention comprises an electric structure of threecomponents: the compound semiconductor layer, i.e., the contact layer;the insulation layer; and the electrode, all of which are disposed onthe light emitting surface of the upper DBR, as shown in FIG. 14A. Theelectric structure also provide optical functions.

The structure on the upper DBR will be described in relation to opticalelements. As shown in FIG. 14B, the contact layer has the first opening,and the insulation layer has the third opening. The contact layerextending in a ring shape and the insulation layer extending in a ringshape on the contact layer are formed step-wise. The complex refractiveindex becomes great from the center of the first opening, i.e., thecenter of the light-emitting surface towards the outside, whereby thecomplex refractive index distribution structure acts as a concave lens.

The electrode made of the metal film is formed on the light-emittingsurface, and has the second opening that is smaller than the firstopening. The electrode has an aperture through which the light passes,whereby the complex refractive index distribution structure acts as aconvex lens as well as an absorption opening, with the complexrefractive index of the metal taken into consideration.

In other words, in the surface light emitting semiconductor laserelement according to one aspect of the present invention, a combinedoptical system of the convex lens, the absorption opening and theconcave lens is provided on the light-emitting surface. In addition, thecombined optical system is disposed on a resonator of the surface lightemitting semiconductor laser element and thus acts as one part of theresonator.

In the surface light emitting semiconductor laser element according toone aspect of the present invention, laser resonance modes are selectedto some extent by the current confinement layer. Light in the high-ordermode having a wide light-emitting angle is scattered at the concavelens, absorbed in the absorption opening and converged in the convexlens, as shown in FIG. 14B. The resonance conditions of the resonatorare determined based on such mechanism. By combining the conditions withthe effects of the aperture of the current confinement layer, almost onemode is forcedly selected, thereby oscillating at the single-peaktransverse mode.

According to the spirit of one aspect of the present invention, thesurface light emitting semiconductor laser element can be controlled invarious transverse modes, i.e., the high-order mode.

According to another aspect of the present invention, the island-likemetal film is disposed within the annular metal film, and the shape ofthe island-like metal film is changed based on the same spirit in oneaspect of the present invention to adjust the complex refractive indexdistribution, whereby the surface light emitting semiconductor laserelement can be controlled in various transverse modes, i.e., thehigh-order mode as desired.

The compound semiconductor layer having the first opening comprises aplurality of layers having different impurity concentrations,

each of the first openings disposed on respective compound semiconductorlayers has a diameter that becomes smaller step-wise from an upper layerto a lower layer of the plurality of compound semiconductor layers, and

each of the impurity concentrations of respective compound semiconductorlayers gradually decreases step-wise from the upper layer to the lowerlayer of the plurality of compound semiconductor layers.

Typically, the metal film constitutes an electrode, and the compoundsemiconductor layer constitutes a contact layer in ohmic contact withthe metal film.

Preferably, the current confinement layer has a non-oxidized currentinjection area at the center, and the non-oxidized current injectionarea is disposed under the first opening, has an impurity concentrationof 5×10¹⁸ cm⁻³, and has uniform current injection density. Theabove-mentioned combined optical system can be efficiently act as onepart of the resonator.

A method of producing a surface light emitting semiconductor laserelement of the present invention comprises the steps of:

sequentially laminating a lower reflector including a semiconductormulti-layer, an active layer, an upper reflector including asemiconductor multi-layer having a layer with a high Al content, and acontact layer on a substrate,

etching the upper reflector having the layer with the high Al content toform a mesa post,

forming an insulation film on the contact layer of the mesa post and aside of the mesa post,

forming an opening on the insulation film over the contact layer toexpose the contact layer,

forming an opening on the contact layer smaller than the opening on theinsulation film to expose the upper reflector,

forming a metal film for constituting an electrode on the upperreflector and the contact layer, and

forming an opening on the metal film smaller than the opening on thecontact film to expose the upper reflector.

In the step of forming the contact layer on the upper reflection layer,a plurality of contact layers are formed so that each of the impurityconcentrations decreases step-wise or gradually from the upper layer tothe lower layer.

In the step of forming the opening on the contact layer smaller than theopening on the insulation film to expose the upper reflector, theopening is formed on each contact layer so that each opening diameterdecreases step-wise or gradually from the upper layer to the lower layerby utilizing a difference in etching rates by the fact that each of theimpurity concentrations decreases step-wise or gradually from the upperlayer to the lower layer. Thus, the complex refractive indexdistribution structure can be easily formed.

In the step of forming the contact layer on the upper reflection layer,a plurality of contact layers are formed so that each Al compositiondecreases step-wise or gradually from the upper layer to the lowerlayer.

In the step of forming the opening on the contact layer smaller than theopening on the insulation film to expose the upper reflector, theopening is formed on each contact layer so that each opening diameterdecreases step-wise or gradually from the upper layer to the lower layerby utilizing a difference in etching rates by the fact that each Alcomposition decreases step-wise or gradually from the upper layer to thelower layer. Thus, the complex refractive index distribution structurecan be easily formed.

According to one aspect of the present invention, a surface lightemitting semiconductor laser element for emitting laser light in asingle-peak transverse mode can be provided by forming a complexrefractive index distribution structure composed of an annular metalfilm and the compound semiconductor layer on an upper reflector where acomplex refractive index is changed from the center of an opening of themetal film, i.e., a center of a light emitting surface towards theoutside.

When the surface light emitting semiconductor laser element according toone aspect of the present invention is used, a combined optical systemconnected to optical fibers and an optical waveguide can besignificantly simplified. In addition, the surface light emittingsemiconductor laser element of the present invention has a smalllight-emitting angle as compared with a conventional end face radiationtype laser element, whereby the surface light emitting semiconductorlaser element of the present invention can be connected to the opticalfibers with high optical connection efficiency.

The surface light emitting semiconductor laser element for emittinglaser light in a single-peak transverse mode of the present inventioncan be connected to quartz single mode fibers that is difficult for theconventional surface light emitting semiconductor laser element. Forexample, when the surface light emitting semiconductor laser element ofthe present invention is used in long wavelength bands such as infrared1.3 μm band and 1.55 μm band, a long distance transmission, i.e., overtens kilometer, can be realized.

When the surface light emitting semiconductor laser element according toone aspect of the present is used in an optical wiring field where thecombined optical system is hardly used in view of the costs, it ispossible to provide a direct connection with high efficiency. Thus, thesurface light emitting semiconductor laser element according to oneaspect of the present can be used efficiently.

According to another aspect of the present invention, a surface lightemitting semiconductor laser element for emitting laser light in adesired high-order transverse mode can be provided by forming a complexrefractive index distribution structure composed of an annular metalfilm, an island-like metal film, and an annular compound semiconductorlayer on an upper reflector where a complex refractive index is changedfrom the center of an opening of the annular metal film, i.e., a centerof a light emitting surface towards the outside.

The surface light emitting semiconductor laser element according to theanother aspect of the present invention can be advantageously applied tovarious fields including a medical, a machining, or a sensor fields thatrequire various light emission patterns.

According to the present invention, there is also provided a preferablemethod of producing the surface light emitting semiconductor laserelement of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing the structure of a surface lightemitting semiconductor laser element according to a first embodiment ofthe present invention;

FIG. 2 is a top view of the surface light emitting semiconductor laserelement in FIG. 1;

FIG. 3A is a schematic sectional view showing a main part of the surfacelight emitting semiconductor laser element in according to a firstembodiment of the present invention;

FIG. 3B is a schematic sectional view for illustrating functions of themain part corresponding to FIG. 3A;

FIG. 4 is a graph showing a far-field pattern (FFP) of a surface lightemitting semiconductor laser element in according to a first embodimentof the present invention;

FIG. 5A is a sectional view showing a step of manufacturing a surfacelight emitting semiconductor laser element in according to a secondembodiment of the present invention;

FIG. 5B is a sectional view showing a step of manufacturing the surfacelight emitting semiconductor laser element in according to a secondembodiment of the present invention;

FIG. 6C is a sectional view showing a step of manufacturing the surfacelight emitting semiconductor laser element in according to a secondembodiment of the present invention;

FIG. 6D is a sectional view showing a step of manufacturing the surfacelight emitting semiconductor laser element in according to a secondembodiment of the present invention;

FIG. 7E is a sectional view showing a step of manufacturing the surfacelight emitting semiconductor laser element in according to a secondembodiment of the present invention;

FIG. 7F is a sectional view showing a step of manufacturing the surfacelight emitting semiconductor laser element in according to a secondembodiment of the present invention;

FIG. 8 is a sectional view showing the structure of a surface lightemitting semiconductor laser element in according to a third embodimentof the present invention;

FIG. 9 is a sectional view showing the structure of the surface lightemitting semiconductor laser element in according to a third embodimentof the present invention;

FIG. 10A is a sectional view showing the structure of a surface lightemitting semiconductor laser element in according to a fourth embodimentof the present invention;

FIG. 10B is a plan view showing the structure of the surface lightemitting semiconductor laser element in according to a fourth embodimentof the present invention;

FIG. 10C is a waveform of a transverse mode in according to a fourthembodiment of the present invention;

FIG. 11 is a sectional view showing the structure of a surface lightemitting semiconductor laser element in a Comparative Example;

FIG. 12 is a sectional view showing the structure of a conventionalsurface light emitting semiconductor laser element;

FIG. 13A is a sectional view showing a step of manufacturing theconventional surface light emitting semiconductor laser element;

FIG. 13B is a sectional view showing a step of manufacturing theconventional surface light emitting semiconductor laser element;

FIG. 14A is a schematic sectional view showing a main part of a surfacelight emitting semiconductor laser element according to one embodimentof the present invention; and

FIG. 14B is a schematic sectional view illustrating a function of themain part shown in FIG. 14A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be described in more detail which referring to theattached drawing. The conductivity type, the film type, the filmthickness, the film forming method, the size and the like cited in thefollowing embodiments are offered to aid in understanding of the presentinvention and are not to be construed as limiting the scope thereof.

Embodiment 1

FIG. 1 shows a sectional view of a surface light emitting semiconductorlaser element according to the present invention. FIG. 2 is a top viewof the surface light emitting semiconductor laser element. FIG. 3A is aschematic sectional view showing a main part of the surface lightemitting semiconductor laser element. FIG. 3B is a schematic sectionalview for illustrating functions of the main part corresponding to FIG.3A.

As shown in FIG. 1, a surface light emitting semiconductor laser element10 comprises a laminated structure sequentially comprising an n-typeGaAs substrate 12, a lower diffractive bragg reflector (hereinafter“lower DBR”) 14 comprising an n-type semiconductor multi-layer, anAl_(0.3)Ga_(0.7)As lower clad layer 16, a GaAs light emitting layer(active layer) 18, an Al_(0.3)Ga_(0.7)As upper clad layer 20, an upperdiffractive bragg reflector 22 (hereinafter “upper DBR”) comprising ap-type GaAs cap layer, and a p-type GaAs contact layer 24 with a filmthickness of 150 nm having an impurity concentration of 5×10¹⁸ cm⁻³.

The lower DBR 14 has a semiconductor multi-layer structure with a totalfilm thickness of about 4 μm including 35 pairs of n-type AlAs layersand n-type GaAs layers. The upper DBR 22 has a semiconductor multi-layerstructure with a total film thickness of about 3 μm including 25 pairsof p-type Al_(0.9)Ga_(0.1)As layers and p-type Al_(0.1)Ga_(0.9)Aslayers.

A cylindrical mesa post 26 having a mesa diameter of 40 μm is formed byetching the contact layer 24, the upper DBR 22, the upper clad layer 20,the active layer 18, the lower clad layer 16, and the lower DBR 14, asshown in FIGS. 1 and 2.

On the active layer 18 in the upper DBR 22, an oxidized currentconfinement layer 28 is disposed instead of the p-typeAl_(0.9)Ga_(0.1)As layer. The AlAs layer 28 has a film thickness of 30nm, and comprises a circular AlAs layer 28A having a diameter of 12 μmdisposed at the center and an oxidized-Al layer 28B disposed around thecircular AlAs layer 28A.

The AlAs layer 28A is a p-type AlAs layer formed instead of the p-typeAl_(0.9)Ga_(0.1)As layer. The oxidized-Al layer 28B is formed byselectively oxidizing Al in the p-type AlAs layer. The oxidized-Al layer28B has high electrical resistance and functions as a currentconfinement area, while the circular AlAs layer 28A functions as acurrent injection area having electrical resistance lower than that ofthe oxidized-Al layer 28B.

On the mesa post 26, the contact layer 24 has a first opening 30 havingan inner diameter of 20 μm at the center. The contact layer 24 isannular to expose the upper DBR 22 through the first opening 30.

An insulation layer, i.e., a SiO₂ film 32 having a film thickness of 300nm, is extended over the periphery of the contact layer 24, the side ofthe mesa post 26, and the lower DBR 14. The SiO₂ film 32 on the contactlayer 24 has a circular third opening 34 having an inner diameter of 35μm that is greater than the first opening 30 to expose the contact layer24.

A p-side electrode 36 comprising a Ti/Pt/Au metal lamination film havinga film thickness of 500 nm is extended over the upper DBR 22, thecontact layer 24, and the SiO₂ film 32, and has a circular secondopening 38 having an inner diameter of 14 μm on the upper DBR 22 toexpose the upper DBR 22.

As shown in FIG. 2, the AlAs layer (current injection area) 28A hasslightly smaller diameter than the third opening 38 of the p-sideelectrode 36. The AlAs layer 28A has a diameter of 12 μm, and the p-sideelectrode has an inner diameter of 14 μm.

At an opposite surface of the n-type GaAs substrate 12, an n-sideelectrode 40 comprising AuGe/Ni/Au is formed.

FIG. 3 schematically shows optical elements of the upper DBR 22. In thesurface light emitting semiconductor laser element 10, the contact layer24, the SiO₂ film 32, and the p-side electrode 36 on the upper DBR 22provide both electrical and optical functions.

As shown in FIG. 3A, the contact layer 24 having the first opening 30extended in a ring shape and the SiO₂ film 32 having the third opening34 extended in a ring shape on the contact layer 24 are formedstep-wise. Accordingly, the complex refractive index is increasedisotropically from the center of the first opening 30, i.e., the centerof a light emitting surface, towards the outside. As shown in FIG. 3B,there is provided a complex refractive index distribution structure thatacts as a convex lens.

The p-side electrode 36 having the second opening 38 has an aperturethrough which the light passes. As shown in FIG. 3B, the p-sideelectrode 36 provides optical functions similar to the complexrefractive index distribution structure having an absorption opening 44and a convex lens 46, since the metal in the p-side electrode 36provides the complex refractive index.

For example, gold (Au) has a real-part refractive index of 0.2 and animaginary-part (absorption coefficient) refractive index of 5.6 for alaser light with a wavelength of 0.85 μm.

In the surface light emitting semiconductor laser element 10, thecontact layer 24 having the first opening 30 has a refractive indexgreater than that of the opening. The p-side electrode 36 having thesecond opening 38 has an absorption coefficient greater than that of theopening.

A combined optical system of the convex lens 46, the absorption opening44, and the concave lens 42 is provided on the light-emitting surface.In addition, the combined optical system is disposed on a resonator ofthe surface light emitting semiconductor laser element 10 and thus actsas one part of the resonator.

In the surface light emitting semiconductor laser element 10, laserresonance modes are selected to some degree by the current confinementaction of the current confinement layer 28. Light in the high-order modehaving a wide light-emitting angle is scattered at the concave lens 42,absorbed in the absorption opening 44, and converged in the convex lens46, as shown in FIG. 3B.

By combining these conditions with the effects of the aperture of thecurrent confinement layer 28, almost one mode is forcedly selected,thereby oscillating in a single-peak transverse mode.

When the optical output is increased, almost one mode is forcedlyselected by the convex lens 46, the absorption opening 44, and theconcave lens 42, as well as by the aperture of the current confinementlayer 28, whereby multiple transverse modes become a single-peaktransverse mode, even if light is oscillated in the multiple transversemodes.

The full width at half maximum (FWHM) of the surface light emittingsemiconductor laser element 10 produced using the method described belowwas measured. As shown in FIG. 4, the FWHM is 5.5°, which is half orless that of the conventional surface light emitting semiconductor laserelement having a constriction diameter of about 4 μm. Thus, the surfacelight emitting semiconductor laser element 10 is in a single-peaktransverse mode. FIG. 4 is a graph showing a far-field pattern (FFP) ofthe surface light emitting semiconductor laser element 10. In the graph,H and V waveforms are intensity distributions of irradiated beams inplanes orthogonal to each other.

In EMBODIMENT 1, the contact layer 24, the SiO₂ film 32 and the p-sideelectrode 36 are formed step-wise, whereby a complex refractive indexchanging from the center of the second opening 38, i.e., the center of alight emitting surface, towards the outside is formed to provide asingle-peak transverse mode.

The surface light emitting semiconductor laser element 10 can providealmost the same level of optical output as that provided by aconventional multi-mode surface light emitting semiconductor laserelement. Since the surface light emitting semiconductor laser element 10has the same electrical structure as that of the conventional multi-modesurface light emitting semiconductor laser element, the surface lightemitting semiconductor laser element 10 has almost the same level ofresistance and impedance.

The surface light emitting semiconductor laser element 10 emits laserlight in a single-peak transverse mode so that the surface lightemitting semiconductor laser element 10 can be optically coupled toactual optical fibers with high optical-connection efficiency.

Embodiment 2

FIGS. 5A, 5B, 6C, 6D, 7E and 7F are sectional views showing steps ofmanufacturing the surface light emitting semiconductor laser elementaccording to the present invention.

As shown in FIG. 5A, a lower DBR 14, a lower clad layer 16, a lightemitting layer (active layer) 18, an upper clad layer 20, an upper DBR22, and a p-type GaAs contact layer 24 are sequentially laminated on ann-type GaAs substrate 12 using a MOCVD method or the like.

Before the upper DBR 22 is formed, an AlAs layer 28 having a filmthickness of 30 nm is formed instead of the p-type Al_(0.9)Ga_(0.1)Aslayer on the layer of the upper DBR 22 at the nearest side of the activelayer 18.

As shown in FIG. 5B, the contact layer 24, the upper DBR 22, the upperclad layer 20, the active layer 18, the lower clad layer 16, and thelower DBR 14 are etched by a dry etching method using a chlorine-basedgas to form a cylindrical mesa post 26 having a mesa diameter of 40 μm.

The laminated structure having the mesa post 26 is heated at 400° C.under steam atmosphere to selectively oxidize only Al in the AlAs layer28 from the peripheral to the internal side of the mesa post 26, leavinga circular AlAs layer 28A having a diameter of 12 μm at the center, anddisposing an oxidized-Al layer 26B around the AlAs layer 28A. Thus, acurrent confinement layer is formed.

As shown in FIG. 6C, a SiO₂ film 32 is formed over the contact layer 24of the mesa post 26, the side of the mesa post 26, and the lower DBR 14.

As shown in FIG. 6D, the SiO₂ film 32 is etched to provide an opening 34having an inner diameter of 35 μm.

As shown in FIG. 7E, the contact layer 24 exposed on the opening 34 isetched to provide an opening 34 having an inner diameter of 20 μm.

As shown in FIG. 7F, a Ti/Pt/Au metal lamination film 39 is formed onthe mesa post 26.

Furthermore, the metal lamination film 39 is etched to provide anopening 38, whereby a p-side electrode 36 is formed. After the n-typeGaAs substrate 12 is polished to a predetermined thickness, an n-sideelectrode 40 is formed on the opposite surface of the n-type GaAssubstrate 12. Thus, the surface light emitting semiconductor laserelement 10 shown in FIG. 1 can be produced.

As described above, the surface light emitting semiconductor laserelement 10 can be produced with similar processes to those used for theconventional surface light emitting semiconductor laser element exceptfor the sizes of the contact layer 24 and the p-side electrode 36.

Embodiment 3

FIG. 8 shows a sectional view of an alternative surface light emittingsemiconductor laser element according to the present invention.

The alternative surface light emitting semiconductor laser element has asimilar structure in a main part 50 to the surface light emittingsemiconductor laser element 10 except that a contact layer 52 and ap-side electrode 54 have different structures.

As shown in FIG. 8, the contact layer 52 includes three layer: an uppercontact layer 52A, a middle contact layer 52B, and a lower contact layer52C. The impurity concentrations of respective contact layers graduallydecrease step-wise from the upper contact layer to the lower contactlayer.

The lower contact layer 52C has, for example, an impurity concentrationof 5×10¹⁸, which is the lowest among the three contact layers, and hasan opening 56C which is the largest opening. The middle contact layer52B has, for example, an impurity concentration of 1×10¹⁹, which ishigher than the lower contact layer, but lower than the upper contactlayer, and has an opening 56B which is smaller than the opening 56C ofthe lower contact layer, but greater than an opening 56A of the uppercontact layer. The upper contact layer 52A has, for example, an impurityconcentration of 3×10¹⁹, which is the highest among the three contactlayers, and has the opening 56A which is the smallest among the threecontact layers.

The p-side electrode 54 is also formed step-wise so as to conform to thecontact layers 52A, 52B and 52C, as well as the openings 56A, 56B and56C.

According to the configuration of the contact layer 52 and the p-sideelectrode 54, an effective complex refractive index distributionstructure is formed to improve focusing of the light, whereby asingle-peak transverse mode can be more easily provided.

As described above, the contact layer 52 is formed such that threelayers have respective openings in a step-wise fashion. Specifically, anetching mask 58 is disposed on the upper contact layer 52A having lowerimpurity concentration, as shown in FIG. 9. The three contact layers52A, 52B and 52C are dry etched under the same etching conditions. Sincethe etching rates are different due to the different impurityconcentrations, the openings 56A, 56B and 56C having the desired sizesare formed on the three contact layers 52A, 52B and 52C.

Alternatively, the three contact layers may be formed so that the Alcompositions decrease step-wise from the upper contact layer to thelower contact layer. The three contact layers 52A, 52B and 52C are dryetched under the same etching conditions. Since the etching rates aredifferent due to the different Al compositions, the openings 56A, 56Band 56C having diameters that become smaller step-wise from the uppercontact layer to the lower contact layer are formed on the three contactlayers 52A, 52B and 52C.

Embodiment 4

FIGS. 10A and 10B are a sectional view and a plan view respectivelyshowing the structure of a surface light emitting semiconductor laserelement oscillating in a higher-order mode according to the presentinvention. FIG. 10C is a waveform of a transverse mode.

The surface light emitting semiconductor laser element emits light in aTE₀₁ mode (donut-like light emission pattern). As shown in FIGS. 10A and10B, the surface light emitting semiconductor laser element comprises,as a main part 60, a p-side electrode 62 including a circular centralelectrode 64 and an annular electrode 68 disposed via an annular lightemitting window 66, as in EMBODIMENT 1.

The surface light emitting semiconductor laser element has a similarstructure to the surface light emitting semiconductor laser element 10in EMBODIMENT 1 except that the p-side electrode 62 has a differentstructure.

The contact layer 24 and the p-side electrode 62 provide the sameeffects as the complex refractive index distribution structure describedin the surface light emitting semiconductor laser element 10 oscillatingin the single mode. The single basic mode lower than the desiredhigh-order mode is suppressed, and at the same time, modes higher thanthe desired high-order mode are suppressed.

In this EMBODIMENT, the basic mode is absorbed and suppressed at thecircular central electrode 64 made of gold disposed at the center of thelight emitting surface. The modes higher than the TE₀₁ mode arescattered using the aperture of the current confinement layer 28 (seeFIG. 1) and the concave lens of the contact layer 24 in the complexrefractive index distribution structure. Thus, light is selectivelyemitted in the TE₀₁ mode.

As long as the constriction diameter of the current confinement layer isset to cut-off the transverse modes other than the TE₀₁ mode, theselectivity of the TE₀₁ mode is further improved.

As to conventional high-order mode control, Japanese Unexamined PatentApplication Publication No. 2002-359432 discloses, for example, a methodof selecting a mode by forming a groove (or a convex-concave shape)having a depth of a ½ wavelength or ¼ wavelength on a mesa surface toexclude any undesirable excited modes or to include the desirable modes.

However, although some functions can be added to the mesa using postprocessing such as ion beam etching, the devices are processed onlyone-by-one, thus reducing production efficiency, and the groove depth,that is the interference optical path difference, should be preciselydefined, even if the device is subjected to patterning etching.Accordingly, such a conventional semiconductor laser may not beapplicable to commercial devices.

In contrast, the laser resonance mode can be selected by providing thecomplex refractive index distribution structure on the uppermost side ofthe resonator according to the present invention. In addition, thecomplex refractive index distribution structure can be provided byadjusting the shape or the refractive index of the compoundsemiconductor layer on the mesa, the insulation film, or the electrodein the typical production processes without adding any steps. Respectiveparts of the complex refractive index distribution structure can beproduced with such a precision that is required for typical surfacelight emitting semiconductor laser elements. No high precise productionprocesses are required. Currently available general process precision isenough for producing the complex refractive index distribution structureaccording to the present invention. Therefore, the complex refractiveindex distribution structure can be produced with good reproducibility.

Comparative Embodiment

FIG. 11 is a sectional view showing the structure of a comparativesurface light emitting semiconductor laser element.

The comparative surface light emitting semiconductor laser elementcomprises, as a main part 70, a scattering structure that randomlyscatters light to an upper surface of a mesa, and a contact layer 72having a fine convex-concave surface.

Scattering at the convex-concave surface of the contact layer 72 affectsthe oscillation mode. A number of modes oscillate randomly. The lightemitted therefrom includes a number of modes, resulting in a randomlight emission pattern.

What is claimed is:
 1. A surface light emitting semiconductor laserelement, comprising: a substrate, a lower reflector, including asemiconductor multi-layer, disposed on the substrate, an active layerdisposed on the lower reflector, an upper reflector, including asemiconductor multi-layer, disposed on the active layer, a compoundsemiconductor layer having a first opening for exposing the upperreflector and extending over the upper reflector, and a metal filmhaving a second opening for exposing the upper reflector disposed insideof the first opening and extending over the compound semiconductorlayer, wherein the metal film and the compound semiconductor layerconstitute a complex refractive index distribution structure where acomplex refractive index is changed from the center of the secondopening towards the outside.
 2. A surface light emitting semiconductorlaser element according to claim 1, wherein in the complex refractiveindex distribution structure, the complex refractive index is changedisotropically from the center of the second opening towards the outside.3. A surface light emitting semiconductor laser element, comprising: asubstrate, a lower reflector, including a semiconductor multi-layer,disposed on the substrate, an active layer disposed on the lowerreflector, an upper reflector, including a semiconductor multi-layer,disposed on the active layer, a compound semiconductor layer having afirst opening for exposing the upper reflector and extending over theupper reflector, and a metal film including an annular film and anisland-like film, the annular film having a second opening for exposingthe upper reflector being disposed inside of the first opening, theannular film extending over the compound semiconductor layer, and theisland-like film being disposed like islands on the upper reflectorwithin the second opening, wherein the metal film and the compoundsemiconductor layer constitute a complex refractive index distributionstructure where a complex refractive index is changed from the center ofthe second opening towards the outside.
 4. A surface light emittingsemiconductor laser element according to claim 1, further comprising aninsulation film having a third opening disposed outside of the firstopening for exposing the compound semiconductor layer, interposedbetween the compound semiconductor layer and the metal film, wherein themetal film, the compound semiconductor layer, and the insulation filmconstitute a complex refractive index distribution structure where acomplex refractive index is changed from the center of the secondopening towards the outside.
 5. A surface light emitting semiconductorlaser element according to claim 3, further comprising an insulationfilm having a third opening disposed outside of the first opening forexposing the compound semiconductor layer, interposed between thecompound semiconductor layer and the metal film, wherein the metal film,the compound semiconductor layer, and the insulation film constitute acomplex refractive index distribution structure where a complexrefractive index is changed from the center of the second openingtowards the outside.
 6. A surface light emitting semiconductor laserelement according to claim 1, wherein the compound semiconductor layerhaving the first opening comprises a plurality of layers havingdifferent impurity concentrations, each of the first openings disposedon respective compound semiconductor layers has a diameter that becomessmaller step-wise from an upper layer to a lower layer of the pluralityof compound semiconductor layers, and each of the impurityconcentrations of respective compound semiconductor layers graduallydecreases step-wise from the upper layer to the lower layer of theplurality of compound semiconductor layers.
 7. A surface light emittingsemiconductor laser element according to claim 3, wherein the compoundsemiconductor layer having the first opening comprises a plurality oflayers having different impurity concentrations, each of the firstopenings disposed on respective compound semiconductor layers has adiameter that becomes smaller step-wise from an upper layer to a lowerlayer of the plurality of compound semiconductor layers, and each of theimpurity concentrations of respective compound semiconductor layersgradually decreases step-wise from the upper layer to the lower layer ofthe plurality compound semiconductor layers.
 8. A surface light emittingsemiconductor laser element according to claim 1, wherein the metal filmconstitutes an electrode, and the compound semiconductor layerconstitutes a contact layer in ohmic contact with the metal film.
 9. Asurface light emitting semiconductor laser element according to claim 1,further comprising a current confinement layer adjacent to the upperreflector or the active layer on the lower reflector, and which isformed as a mesa post.
 10. A surface light emitting semiconductor laserelement according to claim 1, wherein a current confinement layer has anon-oxidized current injection area at the center, and wherein thenon-oxidized current injection area is disposed under the first opening,has an impurity concentration of 5×10¹⁸ cm⁻³, and has uniform currentinjection density.
 11. A method of producing a surface light emittingsemiconductor laser element, comprising the steps of: sequentiallylaminating a lower reflector including a semiconductor multi-layer, anactive layer, an upper reflector including a semiconductor multi-layerhaving a layer with a high Al content, and a contact layer on asubstrate, etching the upper reflector having the layer with the high Alcontent to form a mesa post, forming an insulation film on the contactlayer of the mesa post and a side of the mesa post, forming an openingon the insulation film over the contact layer to expose the contactlayer, forming an opening on the contact layer smaller than the openingof the insulation film to expose the upper reflector, forming a metalfilm for constituting an electrode on the upper reflector and thecontact layer, and forming an opening on the metal film smaller than theopening on the contact film to expose the upper reflector.
 12. A methodof producing a surface light emitting semiconductor laser elementaccording to claim 11, further comprising the steps of: oxidizing thelayer with the high Al content of the mesa post under steam to leave acenter area of the layer with high Al content as a first currentinjection area including a layer with an non-oxidized high Al content,after the step of forming the mesa post, and forming a currentconfinement area comprising an oxidized-Al layer surrounding the currentinjection area.
 13. A method of producing a surface light emittingsemiconductor laser element according to claim 11, wherein in the stepof forming the contact layer on the upper reflection layer, a pluralityof contact layers are formed so that each of the impurity concentrationsdecreases step-wise or gradually from the upper layer to the lowerlayer, and wherein in the step of forming the opening on the contactlayer smaller than the opening of the insulation film to expose theupper reflector, the opening is formed on each contact layer so thateach opening diameter decreases step-wise or gradually from the upperlayer to the lower layer by utilizing a difference in etching rates bythe fact that each of the impurity concentrations decreases step-wise orgradually from the upper layer to the lower layer.
 14. A method ofproducing a surface light emitting semiconductor laser element accordingto claim 11, wherein in the step of forming the contact layer on theupper reflection layer, a plurality of contact layers are formed so thateach Al composition decreases step-wise or gradually from the upperlayer to the lower layer, and wherein in the step of forming the openingon the contact layer smaller than the opening of the insulation film toexpose the upper reflector, the opening is formed on each contact layerso that each opening diameter decreases step-wise or gradually from theupper layer to the lower layer by utilizing a difference in etchingrates by the fact that each Al composition decreases step-wise orgradually from the upper layer to the lower layer.